Typically, immune cells require a target cell to present antigen via major histocompatibility complex (MHC) before triggering an immune response resulting in the death of the target cell. This allows cancer cells not presenting MHC class I to evade the majority of immune responses.
NK cells are able, however, to recognize cancer cells in the absence of MHC class I expression. Hence they perform a critical role in the body's defense against cancer.
On the other hand, in certain circumstances, cancer cells demonstrate an ability to dampen the cytotoxic activity of NK cells, through expression of ligands that bind inhibitory receptors on the NK cell membrane. Resistance to cancer can involve a balance between these and other factors.
Cytotoxicity, in this context, refers to the ability of immune effector cells, e.g. NK cells, to induce cancer cell death, e.g. by releasing cytolytic compounds or by binding receptors on cancer cell membranes and inducing apoptosis of said cancer cells. Cytotoxicity is affected not only by signals that induce release of cytolytic compounds but also by signals that inhibit their release. An increase in cytotoxicity will therefore lead to more efficient killing of cancer cells, with less chance of the cancer cell dampening the cytotoxic activity of the NK, as mentioned above.
Genetic modification to remove inhibitory receptor function on NK cells has been suggested as a method for increasing the cytotoxicity of NK cells against cancer cells that lack MHC class I expression but are able to dampen NK cytotoxicity (Bodduluru et al. 2012). NKG2A has been established as an inhibitory receptor worth silencing under these circumstances, as certain cancer cells are known to express MICA which binds NKG2A and inhibits NK cell cytotoxicity in the absence of MHC class I expression (Shook et al. 2011; WO 2006/023148).
Another method of downregulating NKG2A expression has been shown in NK-92 cells, in which transfection with a gene encoding IL-15 was shown to be associated with a reduction in NKG2A expression (Zhang et al. 2004). However, despite an observed increase in the cytotoxicity of the NK cells, the increase was likely a result of a concomitant increase in expression of the activating receptor NKG2D. This is supported by the observation that blocking NKG2A receptors on NK-92 cells was not associated with an increase in cytotoxicity against multiple myeloma cells (Heidenreich et al. 2012). Nevertheless, it is worth noting that the NK-92 cell line is a highly cytotoxic cell line with very low expression of inhibitory receptors. Therefore, any increase in cytotoxicity associated with decreased NKG2A expression might have been too trivial to detect.
Similar studies have been carried out in mice. For example, mice express a receptor called Ly49 on NK cells, which is analogous to human inhibitory KIR receptors. It has been shown that by blocking the Ly49 receptor with antibody fragments, NK cells are more cytotoxic and capable of killing murine leukemia cells in vitro and in vivo (Koh et al. 2001).
It is a consequence of reducing inhibitory receptor function, however, that ‘normal’ cells in the body also become more susceptible to attack by modified NK cells, as the modified NK cells become less capable of distinguishing between ‘normal’ cells and cancer cells. This is a significant disadvantage of reducing ‘classical’ inhibitory receptor function.
Another way in which NK cells are known to kill cancer cells is by expressing TRAIL on their surface. TRAIL ligand is able to bind TRAIL receptors on cancer cells and induce apoptosis of said cancer cells. One speculative approach describes overexpressing TRAIL on NK cells, in order to take advantage of this anti-cancer mechanism (EP1621550). Furthermore, IL-12 has been reported to upregulate TRAIL expression on NK cells (Smyth et al. 2001).
Nevertheless, cancer cells have developed evasive and protective mechanisms for dealing with NK cells expressing TRAIL. Decoy TRAIL receptors are often expressed on cancer cell membranes, and binding of TRAIL to these decoy receptors is unable to induce apoptosis; methods of overcoming such mechanisms have not yet been pursued.
Acute myeloid leukemia (AML) is a hematopoietic malignancy involving precursor cells committed to myeloid development, and accounts for a significant proportion of acute leukemias in both adults (90%) and children (15-20%) (Hurwitz, Mounce et al. 1995; Lowenberg, Downing et al. 1999). Despite 80% of patients achieving remission with standard chemotherapy (Hurwitz, Mounce et al. 1995; Ribeiro, Razzouk et al. 2005), survival remains unsatisfactory because of high relapse rates from minimal residual disease (MRD). The five-year survival is age-dependent; 60% in children (Rubnitz 2012), 40% in adults under 65 (Lowenberg, Downing et al. 1999) and 10% in adults over 65 (Ferrara and Schiffer 2013). These outcomes can be improved if patients have a suitable hematopoietic cell donor, but many do not, highlighting the need for an alternative approach to treatment.
Natural killer (NK) cells are cytotoxic lymphocytes, with distinct phenotypes and effector functions that differ from e.g. natural killer T (NK-T) cells. For example, while NK-T cells express both CD3 and T cell antigen receptors (TCRs), NK cells do not. NK cells are generally found to express the markers CD16 and CD56, wherein CD16 functions as an Fc receptor and mediates antibody dependent cell-mediated cytotoxicity (ADCC) which is discussed below. KHYG-1 is a notable exception in this regard. Despite NK cells being naturally cytotoxic, NK cell lines with increased cytotoxicity have been developed. NK-92 and KHYG-1 represent two NK cell lines that have been researched extensively and show promise in cancer therapeutics (Swift et al. 2011; Swift et al. 2012).
Adoptive cellular immunotherapy for use in cancer treatment commonly involves administration of natural and modified T cells to a patient. T cells can be modified in various ways, e.g. genetically, so as to express receptors and/or ligands that bind specifically to certain target cancer cells. Transfection of T cells with high-affinity T cell receptors (TCRs) and chimeric antigen receptors (CARs), specific for cancer cell antigens, can give rise to highly reactive cancer-specific T cell responses. A major limitation of this immunotherapeutic approach is that T cells must either be obtained from the patient for autologous ex vivo expansion or MHC-matched T cells must be used to avoid immunological eradication immediately following transfer of the cells to the patient or, in some cases, the onset of graft-vs-host disease (GVHD). Additionally, successfully transferred T cells often survive for prolonged periods of time in the circulation, making it difficult to control persistent side-effects resulting from treatment.
In haplotype transplantation, the graft-versus-leukemia effect is believed to be mediated by NK cells when there is a KIR inhibitory receptor-ligand mismatch, which can lead to improved survival in the treatment of AML (Ruggeri, Capanni et al. 2002; Ruggeri, Mancusi et al. 2005). Furthermore, rapid NK recovery is associated with better outcome and a stronger graft-vs-leukemia (GVL) effect in patients undergoing haplotype T-depleted hematopoietic cell transplantation (HCT) in AML (Savani, Mielke et al. 2007). Other trials have used haploidentical NK cells expanded ex vivo to treat AML in adults (Miller, Soignier et al. 2005) and children (Rubnitz, Inaba et al. 2010).
Several permanent NK cell lines have been established, and the most notable is NK-92, derived from a patient with non-Hodgkin's lymphoma expressing typical NK cell markers, with the exception of CD16 (Fc gamma receptor III). NK-92 has undergone extensive preclinical testing and exhibits superior lysis against a broad range of tumors compared with activated NK cells and lymphokine-activated killer (LAK) cells (Gong, Maki et al. 1994). Cytotoxicity of NK-92 cells against primary AML has been established (Yan, Steinherz et al. 1998).
Another NK cell line, KHYG-1, has been identified as a potential contender for clinical use (Suck et al. 2005) but has reduced cytotoxicity so has received less attention than NK-92. KHYG-1 cells are known to be pre-activated. Unlike endogenous NK cells, KHYG-1 cells are polarized at all times, increasing their cytotoxicity and making them quicker to respond to external stimuli. NK-92 cells have a higher baseline cytotoxicity than KHYG-1 cells.
It is therefore clear that current adoptive immunotherapy protocols are affected by donor variability in the quantity and quality of effector cells, variables that could be eliminated if effective cell lines were available to provide more standardized therapy.
A considerable amount of research into NK cell cytotoxicity has been performed using mouse models. One example is the finding that perforin and granzyme B mRNA are constitutively transcribed in mouse NK cells, but minimal levels of protein are detected until stimulation or activation of the NK cells (Fehniger et al, 2007). Although this work and other work using mouse NK cells is of interest, it cannot be relied upon as conclusive evidence for NK cell cytotoxicity in humans. In contrast to the above example, human NK cells express high levels of perforin and granzyme B protein prior to stimulation (Leong et al, 2011). The result being that when either mouse or human NK cells are freshly isolated in culture, the mouse NK cells have weak cytolytic activity, whereas the human NK cells exhibit strong cytolytic capabilities.
Mouse and human NK cells also vary greatly in their expression markers, signalling cascades and tissue distribution. For example, CD56 is used as a marker for human NK cells, whereas mouse NK cells do not express this marker at all. Furthermore, a well-established mechanism for regulating NK cell cytotoxicity is via ligand binding NK activation and inhibitory receptors. Two of the most prominent human NK activation receptors are known to be NKp30 and NKp44, neither of which are expressed on mouse NK cells. With regards to NK inhibitory receptors, whilst human NK cells express KIRs that recognise MHC class I and dampen cytotoxic activity, mouse NK cells do not express KIRs at all but, instead, express Ly49s (Trowsdale et al, 2001). All in all, despite mouse NK cells achieving the same function as human NK cells in their natural physiological environment, the mechanisms that fulfil this role vary significantly between species.
Thus there exists a need for alternative and preferably improved human NK cells and human NK cell lines, e.g. with a more cytotoxic profile.                An object of the invention is to provide NK cells and NK cell lines with a more cytotoxic phenotype. A further object is to provide methods for producing modified NK cells and NK cell lines, compositions containing the cells or cell lines and uses of said compositions in the treatment of cancers. More particular embodiments aim to provide treatments for identified cancers, e.g. blood cancers, such as leukemias. Specific embodiments aim at combining two or more modifications of NK cells and NK cell lines to further enhance the cytotoxicity of the modified cells.        